Protective surface coatings are organic compositions applied to substrates and cured to form continuous films primarily to protect the substrate surface. Protective coatings ordinarily comprise a polymeric binder, pigments, and various additives, although some coatings contain little or no pigments and are known as clear coatings.
Epoxy resins are particularly desirable for use in surface coating materials as a vehicle or polymeric binder, where the epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are especially desirable for interior can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such sanitary can coatings must not alter the product taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching coating components into the beverage, or by adsorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof.
In commonly assigned U.S. Pat. No. 4,212,781, a non-aqueous process is disclosed for modifying epoxy resin by reacting the epoxy resin with addition polymerizable monomer in the presence of at least 3% by weight of benzoyl peroxide (or the free radical peroxide initiating equivalent) based on monomer at a suitable reaction temperature. This reaction produces a non-aqueous reaction mixture containing an in-situ formed blend of resinous materials comprising an epoxy-acrylic copolymer mixture containing epoxy resin, a carbon graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. The in-situ polymerized monomers include carboxylic acid functional monomers to provide sufficient acid functionality in the reaction mixture to enable stable dispersion of the resulting reaction product in a basic aqueous medium.
To provide highly desirable epoxy-derived protective coatings, low molecular weight liquid epoxy resins are advanced in molecular weight to produce a high molecular weight epoxy resin preferably having a molecular weight above about 7000 and higher. High molecular weight epoxy resins are generally made by a fusion or organic solvent process in which low molecular weight epoxy resin, such as the diglycidyl ether of bisphenol A (DGEBPA), is coreacted with a lesser amount of bisphenol A to give a high molecular weight epoxy resin. For instance, a typical low molecular weight liquid epoxy resin having a molecular weight of about 380 can be advanced by reacting with bisphenol A at a weight ratio of about 66/33 weight parts epoxy resin/bisphenol A in about 15 weight parts of ethylene glycol monobutyl ether solvent and at a reaction temperature of about 175.degree. C. in the presence of an ethyl triphenyl phosphonium acetate catalyst at atmospheric pressure, as described in U.S. Pat. No. 4,212,781, to produce a high molecular weight epoxy resin. The high molecular weight epoxy acrylic copolymers can be emulsified into water if first dissolved in an organic solvent to a low viscosity. Solvent levels of 50 to 100 parts per 100 parts of epoxy-acrylic by weight are generally required inasmuch lower solvent levels tend to render the solvent mixture difficult to disperse into water. More typical epoxy suspensions are disclosed in U.S. Pat. No. 4,409,288 and U.S. Pat. No. 4,177,177, which first disperse high molecular weight epoxy resin into solvent which in turn are dispersed into water followed by stripping of the organic solvent. The organic solvent must be removed from the aqueous emulsion by distillation, which is an expensive and time consuming process. In U.S. Pat. No. 4,122,067, a process is disclosed for reacting a polyepoxide with a dihydric phenol in water as a 40% to 95% aqueous dispersion and at temperatures between 80.degree. C. and 100.degree. C. using nonionic surfactants to produce water dispersed very low molecular weight epoxy resins. Dispersion at 40% to 75% are said to be unstable and resulting with large particles settling out of water while 75% to 95% dispersions result in much smaller particles but stable dispersions. Under these conditions, it has been found that mixtures of liquid epoxy and bisphenol coreact and advance the molecular weight only to about 1,000 where the reaction mixture becomes glassy and the reaction stops.
It now has been found that aqueous emulsions of low molecular weight epoxy resin can be advanced in molecular weights with bisphenols such as bisphenol-A while dispersed in water to produce small particle size stable aqueous dispersions of the high molecular weight epoxy resin in water. In accordance with the process of this invention, stabilized dilute aqueous emulsions (10% to 30%) of high molecular weight epoxy resins can be produced by reacting diepoxide with extender diphenol at temperatures of about 110.degree. C. to 150.degree. C. if reacted under pressures above about 10 psig in conjunction with nonionic and anionic stabilizers. The process of this invention provides a convenient and economical method for producing a stable high molecular weight epoxy resin emulsion directly at NV's which are suitable for interior can coating applications without the need for the cumbersome multi-step process of intermediate mixing with high levels of organic solvent, dispersion into water, and subsequent solvent stripping. The stable, high molecular weight, epoxy aqueous emulsion resulting from the process of this invention can be further coreacted directly with ethylenic unsaturated monomers, if desired, to produce epoxy emulsion copolymers suitable for use as a very low VOC (volatile organic compounds) aqueous dispersed coating. A further advantage of this invention is that considerably lesser amounts of acrylic or methacrylic acid monomer can be copolymerized with other ethylenic monomers to maintain water dispersibility and stability of the epoxy-acrylic copolymer in water. A further advantage relates to enhanced rheological properties including improved coating application properties as well as improved performance and sag resistance properties. In a further embodiment, carboxyl functional acrylic acids can be reacted with epoxy groups of advanced molecular weight, aqueous dispersed epoxy resins in intermediate stages to provide terminal double bond functionality which can be used to control molecular grafting during post reactions with acrylic monomer. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.